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GeneralPurpose Mail Filter 
Sergey Poznyakoff 
A boolean expression is a combination of relational or
matching expressions using the boolean operators and
, or
and not
, and, eventually, parentheses to control nesting:
Expression  Result 

x and y  True only if both x and y are true. 
x or y  True if any of x or y is true. 
not x  True if x is false. 
Binary boolean expressions are computed using shortcut evaluation:
x and y
If x ⇒
, the result is false
false
and y is not evaluated.
x or y
If x ⇒
, the result is true
true
and
y is not evaluated.
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